Methods for electroplating copper

ABSTRACT

Embodiments of the invention are directed to methods of electroplating copper onto at least one surface of a substrate in which more uniform electrical double layers are formed adjacent to the at least one surface being electroplated (i.e., the cathode) and an anode of an electrochemical cell, respectively. In one embodiment, the electroplated copper may be substantially-free of dendrites, exhibit a high-degree of (111) crystallographic texture, and/or be electroplated at a high-deposition rate (e.g., about 6 μm per minute or more) by electroplating the copper under conditions in which a ratio of a cathode current density at the at least one surface to an anode current density at an anode is at least about 20. In another embodiment, a porous anodic film may be formed on a consumable copper anode using a long conditioning process that promotes forming a more uniform electrical double layer adjacent to the anode.

BACKGROUND

Copper-based materials have currently supplanted aluminum-basedmaterials as the material of choice for interconnects in integratedcircuits (“ICs”). Copper offers a lower electrical resistivity and ahigher electromigration resistance than that of aluminum, which hashistorically been the dominant material used for interconnects.

Interconnects in ICs are becoming one of the dominant factors fordetermining system performance and power dissipation. For example, thetotal length of interconnects in many currently available ICs can betwenty miles or more. At such lengths, interconnectresistance-capacitance (“RC”) time delay can exceed a clock cycle andseverely impact device performance. Additionally, the interconnect RCtime delay also increases as the size of interconnects continues torelentlessly decrease with corresponding decreases in transistor size.Using a lower resistivity material, such as copper, decreases theinterconnect RC time delay, which increases the speed of ICs that employinterconnects formed from copper-based materials. Copper also has athermal conductivity that is about two times aluminum's thermalconductivity and an electromigration resistance that is about ten toabout one-hundred times greater than that of aluminum.

Copper-based interconnects have also found utility in other applicationsbesides ICs. For example, solar cells, flat-panel displays, and manyother types of electronic devices can benefit from using copper-basedinterconnects for the same or similar reasons as ICs.

Due to difficulties uniformly depositing and void-free filling trenchesand other small features with copper using physical vapor deposition(“PVD”) and chemical vapor deposition (“CVD”), copper interconnects aretypically fabricated using a Damascene process. In the Damasceneprocess, a trench is formed in, for example, an interlevel dielectriclayer, such as a carbon-doped oxide. The dielectric layer is coveredwith a barrier layer formed from, for example, tantalum or titaniumnitride to prevent copper from diffusing into the silicon substrate anddegrading transistor performance. A seed layer is formed on the barrierlayer to promote uniform deposition of copper within the trench. Thesubstrate is immersed in an electroplating solution that includescopper. The substrate functions as a cathode of an electrochemical cellin which the electroplating solution functions as an electrolyte, andthe copper from the electroplating solution or a consumable anode iselectroplated into the trench responsive to a voltage applied betweenthe substrate and an anode. Then, copper deposited on regions of thesubstrate outside of the trench is removed using chemical-mechanicalpolishing (“CMP”).

Regardless of the particular electronic device in which copper is usedas a conductive structure, it is important that an electroplatingprocess for copper be sufficiently fast to enable processing a largenumber of substrates and have an acceptable yield. Additionally, thecost of the electroplating solution is also another factor impactingoverall fabrication cost of electronic devices using copper. This isparticularly important in the fabrication of solar cells, which have tocost-effectively compete with other, potentially more cost-effective,energy generation technologies. Thus, it is desirable that copperelectroplating solutions be capable of depositing copper in a uniformmanner (i.e., high-throwing power) and at a high-deposition rate.

A number of electroplating solutions are currently available forelectroplating copper. For example, sulfate-based electroplatingsolutions are commonly used for electroplating copper. Some alkalinecopper electroplating solutions have a high-throwing power, but are notcapable of rapidly depositing copper without compromising the depositedfilm quality. At high-deposition rates, the copper may grow as dendritesas opposed to a more uniformly deposited film. Additionally, alkalielements (e.g., sodium and potassium) in such alkaline copperelectroplating solutions can diffuse into silicon substrates and aredeep-level impurities in silicon that can compromise transistorperformance. Fluoroborate electroplating solutions can be used forhigh-speed deposition of copper. However, fluoroborate electroplatingsolutions can be more expensive than, more traditional, sulfate-basedsolutions.

Moreover, fluoroborate electroplating solutions may be more hazardousand difficult to dispose of than many other electroplating solutions forelectroplating copper.

SUMMARY

Embodiments of the invention are directed to methods of electroplatingcopper onto at least one surface of a substrate in which more uniformelectrical double layers are formed adjacent to the at least one surfacebeing electroplated (i.e., the cathode) and an anode of anelectrochemical cell, respectively. The electroplated copper may be ofhigh-quality and electroplated at a high-deposition rate so that theelectroplated copper may be used, for example, in electricalinterconnects for ICs, solar cells, and many other applications.

In one embodiment of the invention, a method is disclosed in which theelectroplated copper may be substantially-free of dendrites, exhibit ahigh-degree of (111) crystallographic texture, and/or be electroplatedat a high-deposition rate (e.g., about 6 μm per minute or more) byelectroplating copper under conditions in which a ratio of a cathodecurrent density at the at least one surface of the substrate beingelectroplated to an anode current density at an anode is at least about20. In such an embodiment, the method includes forming anelectrochemical cell comprising at least one surface of a substrate, ananode, and an electroplating solution in contact with the at least onesurface and the anode, wherein the electroplating solution includes atleast one suppressor agent. The method further includes electroplatingcopper onto the at least one surface under conditions in which a ratioof a cathode current density at the surface to an anode current densityat the anode is at least about 20.

In another embodiment of the invention, a method includes forming anelectrochemical cell comprising a cathode, a consumablecopper-containing anode, and an electroplating solution in contact withthe cathode and the consumable copper-containing anode. The methodfurther includes forming a porous anodic film on the consumablecopper-containing anode by generating a current through theelectrochemical cell for a time sufficient to pass at least about 1000coulombs per liter through the electroplating solution. In oneembodiment, the cathode may be a conditioning cathode that is replacedwith a substrate having at least one surface to be electroplated thatfunctions as a cathode. In such an embodiment, the method also includeselectroplating copper onto the at least one surface of the substrate. Inanother embodiment, the consumable copper-containing anode may beconditioned to form the porous anodic film in a separate electrochemicalcell, and subsequently removed and employed in a plating electrochemicalcell in which at least one surface of a substrate to be electroplatedfunctions as the cathode.

Other embodiments of the invention relate to methods of synthesizing anaccelerator agent that is substantially free of alkali elements for usein an electroplating solution.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate various embodiments of the invention, whereinidentical reference numerals refer to identical elements or features indifferent views or embodiments shown in the drawings.

FIG. 1 is a schematic cross-sectional view of an embodiment of anelectroplating apparatus that may be used for electroplating copper ontoat least one surface of a substrate according to embodiments of methodsof the invention.

FIG. 2 is a cross-sectional view of a consumable, copper anode includinga mass of copper particles enclosed in a porous membrane according to anembodiment of the invention.

FIG. 3 is a side elevation view of a consumable, copper anode made fromsintered copper particles according to an embodiment of the invention.

FIG. 4 is an isometric view of a consumable, copper anode comprising abody having a plurality of grooves formed therein according to anembodiment of the invention.

FIGS. 5A-5E are cross-sectional views illustrating various stages in ato method of electroplating bumps according to an embodiment of theinvention.

DETAILED DESCRIPTION

Embodiments of the invention are directed to methods of electroplatingcopper onto at least one surface of a substrate in which more uniformelectrical double layers are formed adjacent to the at least one surfacebeing electroplated (i.e., the cathode) and an anode of anelectrochemical cell, respectively. The electroplated copper may be usedin electrical interconnects for ICs, solar cells, and many otherapplications. For example, in an embodiment, the electroplated coppermay be substantially-free of dendrites, exhibit a high-degree of (111)crystallographic texture, and/or be electroplated at a high-depositionrate (e.g., about 6 μm per minute or more) by electroplating the copperunder conditions in which a ratio of a cathode current density at the atleast one surface to an anode current density at an anode is at leastabout 20. In another embodiment, a porous anodic film may be formed on aconsumable copper anode using a long-conditioning process that promotesforming a more uniform electrical double layer adjacent to the anodeduring electroplating.

Electroplating Apparatuses for Practicing Described ElectroplatingMethods

FIG. 1 is a schematic cross-sectional view of an embodiment of anelectroplating apparatus 100 that may be employed for practicingembodiments of methods of the invention. The electroplating apparatus100 includes a container 102 holding an electroplating solution 104comprising at least one suppressor agent and having a high-surface areaanode 106 immersed therein. Although not shown in FIG. 1, theelectroplating apparatus 100 may include a heating unit configured tomaintain the electroplating solution 104 at a temperature of about 20°C. to about 60° C. and, more specifically, at about 40° C. A substrateholder 108 is configured to hold a substrate 110, having at least onesurface 112, to be electroplated in the electroplating solution 104. Thesubstrate holder 108 may be configured according to any conventional orsubsequently developed substrate holder. Although only a singlesubstrate is illustrated in FIG. 1 for simplicity, many commerciallyavailable substrate holders are configured to hold multiple substrates.Additionally, as used herein, the term “substrate” refers to anyworkpiece capable of being electroplated. For example, suitablesubstrates include, but are not limited to, semiconductor substrates(e.g., single-crystal silicon wafers, single-crystal gallium arsenidewafer, etc.) with or without active and/or passive devices (e.g.,transistors, diodes, capacitors, resistors, etc.) formed therein andwith or without at least one surface coated with a seed layer, printedcircuit boards, flexible polymeric substrates, and many other types ofsubstrates. As will be discussed in more detail below, theelectroplating solution 104 is formulated and the anode 106 isconfigured so that a generally uniform electrical double layer is formedat both the surface 112 being electroplated and the anode 106 so that amore uniform current density is developed at both the surface 112 (i.e.,cathode) and the anode 106.

A power supply 114 is electrically connected to the anode 106 and,through electrical contacts (not shown) in the substrate holder 108, tothe surface 112 of the substrate 110 to be electroplated. The powersupply 114 may be operable to apply a selected voltage waveform betweenthe anode 106 and substrate 110, such as a constant voltage, atime-varying voltage waveform, or both. Thus, the surface 112 of thesubstrate 110 defining a cathode, the anode 106, and the electroplatingsolution 104 form an electrochemical cell 115.

Still referring to FIG. 1, a movable arm 116 is connected to thesubstrate holder 108 and may orient the surface 112 of the substrate 110downwardly, and further is operably connected to an actuator system 118.The surface 112 may be spaced from the anode 106 by a spacing S of about0.5 to about 50 centimeters (“cm”). The actuator system 118 is operableto selectively move the movable arm 116 in directions V₁ and V₂ toenable immersing and removal of the substrate holder 108 carrying thesubstrate 110 from the electroplating solution 108. Additionally, theactuator system 118 may be operable to controllably move the movable arm116 in directions H₁ and H₂ in a linearly oscillatory manner to move thesubstrate 110 in the directions H₁ and H₂, rotate the movable arm 116and the substrate holder 108 connected thereto in a direction R, or bothduring electroplating.

The electroplating apparatus 100 may also include a number of containers(not shown) holding different solutions, such as a cleaning solution,drying solution, rinsing solution, etc. Furthermore, a variety ofdifferent fluid supply systems may be employed to supply the variousfluids in the containers and, optionally, to re-circulate theelectroplating solution 104 to provide a generally laminar flow of theelectroplating solution 104 over the substrate 110. Such fluid supplysystems, container configurations, and cleaning/drying/rinsing solutionsare well-known and in the interest of brevity are not described indetail herein.

Embodiments of Methods for Electroplating Copper

Referring to FIG. 1, according to an embodiment of a method of theinvention, the electrochemical cell 115 is formed by immersing thesubstrate holder 108 carrying the substrate 110 in the electroplatingsolution 104 so that the surface 112 and anode 106 are in contact withthe electroplating solution 104. As will be discussed in more detailbelow, the electroplating solution 104 is formulated with at least onesuppressor agent to promote forming a more uniform electrical doublelayer adjacent to the surface 112 during electroplating. That is, theresistance drop across the electrical double layer in the electroplatingsolution 104 adjacent to the surface 112 is more uniform in a directiongenerally parallel to the surface 112. Additionally, the high-surfacearea anode 106 is configured to provide a more uniform electrical doublelayer adjacent to the anode 106 during electroplating. That is, theresistance drop across the electrical double layer in the electroplatingsolution 104 adjacent to the anode 106 is more uniform in a directionalong the length of and about the anode 106.

A copper film 120 is electroplated onto the surface 112 of the substrate110 under conditions in which a ratio of a cathode current density atthe surface 112 to an anode current density at the anode 106 is at leastabout 20 (e.g., about 20 to about 200) by application of a selectedvoltage waveform between the surface 112 and the anode 106 using thepower supply 114. Furthermore, the ratio may be about 20 to about 100,more particularly about 40 to about 100, even more particularly about 60to about 100, and yet even more particularly about 80 to about 100. Thesurface area of the anode 106 relative to the surface area of thesurface 112 being electroplated, and the suppression strength of the atleast one suppressor agent are selected to that the above ratios may bemaintained during electroplating of the copper film 120. Theelectroplating copper that forms the copper film 120 may be providedfrom the anode 106 when the anode 106 is a consumable anode or may beprovided from copper intentionally added to the electroplating solution104.

During electroplating of the copper film 120, the substrate 110 may belinearly oscillated at a rate of about 10 millimeters per second(“mm/s”) to about 1000 mm/s back and forth in the directions H₁ and H₂.In another embodiment, the substrate holder 108 and substrate 110 may berotated in the direction R as a unit while the surface 112 of thesubstrate 110 is maintained generally parallel to a longitudinal axis ofthe anode 106. For example, the substrate holder 108 and substrate 110may be rotated in the direction R as a unit at a rotational speed ofabout 150 revolutions per minute (“RPM”) to about 300 RPM and, moreparticularly, about 200 RPM. In other embodiments of the invention, acombination of linear oscillatory movement of the substrate holder 108and substrate 110 as a unit in the directions H₁ and H₂ and rotationalmovement in the direction R may be used. Utilizing any of theabove-described techniques for linearly oscillating and/or rotating thesubstrate 110 enables increasing the limiting current density at thesubstrate 110 that is limited by diffusion of cupric ions within theelectroplating solution 104 to the surface 112 of the substrate 110.

During the electroplating process, in one embodiment of the invention,the power supply 114 may apply a generally constant voltage between thesurface 112 of the substrate 110 and the anode 106. In anotherembodiment of the invention, the power supply 114 may apply time-varyingvoltage to impose a forward-pulse current density on the substrate 110to promote forming a finer grain size in the copper film 120.Representative cathode current densities at the surface 112 of thesubstrate 110 (i.e., the cathode) for a forward-pulse current densitywaveform may be about 200 mA/cm² to about 2000 mA/cm², while anodecurrent densities at the anode 106 may be about 10 mA/cm² or less. Inanother embodiment, the power supply 114 may apply a time-varyingvoltage to impose a reverse-pulse current density waveform on thesurface 112 of the substrate 110 or a combination of a forward-pulse andreverse-pulse current density waveform. Representative current densitiesat the surface 112 of the substrate 110 (i.e., the cathode) for theforward pulse of a forward-pulse/reverse-pulse current density waveformmay be increased to about 10 A/cm² with a pulse duration of about 0.1 msto about 100 ms. In the above-described time-varying voltage waveforms,the ratio of the cathodic current density to the anodic current densityis determined by the peak current density at the cathode (i.e., thesurface 112 of the substrate 110) to the corresponding peak currentdensity at the anode 106.

The described embodiments of electroplating the copper film 120 thatutilize the selectively formulated electroplating solution 104 incombination with the high-surface area anode 106 enables electroplatingthe copper film 120 at a high-deposition rate, such as about 6 μm perminute or more and, more particularly, about 9 μm per minute.Additionally, the electroplated copper film 120 may be substantiallyfree of dendrites and exhibit a high-degree of (111) crystallographictexture that is more resistant to stress-induced voiding than othercrystallographic textures.

Embodiments of Consumable Copper Anodes

In an embodiment of the invention, the anode 106 may be a consumable,copper-containing anode. Referring to FIG. 2, in one embodiment, theanode 106 may be configured as a porous mass of copper particles 200enclosed in a suitable polymeric membrane 202 that is permeable tocupric ions (Cu²⁺) and the electroplating solution 104. Referring toFIG. 3, in another embodiment, the anode 106 may be configured as porousmass 300 of sintered-together copper particles. Referring to FIG. 4, inanother embodiment, the anode 106 may be configured as a rod 400 or bodyof other geometry made from copper that includes a plurality of grooves402 formed therein. In yet another embodiment, the anode 106 may beconfigured as a copper mesh. In any of the above-described embodimentsfor the anode 106, the anode 106 provides the copper to be depositedonto the surface 112 of the substrate 110. Application of the selectedvoltage waveform between the anode 106 and the surface 112 of thesubstrate 110 causes copper from the consumable anode 106 to oxidize,dissolve in the electroplating solution 104, and be electroplated ontothe surface 112.

Embodiments of Electroplating Solutions

The electroplating solution 104 may formulated from at least one acidand at least one suppressor agent. In some embodiments of the invention,the electroplating solution 104 may also include at least oneaccelerator agent. The at least one acid may be selected from one ormore of the following acids: sulfuric acid, methane sulfonic acid,hydrochloric acid, hydroiodic acid, hydroboric acid, fluoroboric acid,and any other suitable acid. In a more specific embodiment of theinvention, the at least one acid includes sulfuric acid present in aconcentration of about 100 grams per liter (“g/L”) or less (e.g., about5 g/L to about 100 g/L) and hydrochloric acid present in a concentrationfrom about 20 mg/L to about 100 mg/L. In addition to the aforementionedacids, in certain embodiments of the invention, the electroplatingsolution 104 may further include a supplemental acid selected toincrease the solubility of the copper from the consumable anode 106 inthe at least one acid. For example, the supplemental acid may beselected from alkane sulfonic acid, methane sulfonic acid, ethanesulfonic acid, propane sulfonic acid, buthane sulfonic acid, penthanesulfonic acid, hexane sulfonic acid, decane sulfonic acid, dedecanesulfonic acid, fluoroboric acid, mixtures of any of the precedingsupplemental acids, or another suitable acid selected to increase thesolubility of the copper in the at least one acid of the electroplatingsolution 104.

As discussed above, the electroplating solution 104 may includeadditives, such as a suppressor agent, an accelerator agent, or boththat improve certain electroplating characteristics of theelectroplating solution 104. As used herein, the phrase “virgin makesolution” (“VMS”) refers to an electroplating solution 104 without anysuppressor agents and accelerator agents. For the electroplatingsolution 104 described herein, the VMS includes the at least one acid.As used herein, “suppression strength” of one or more suppressor agentsof an electroplating solution 104 is determined by a decrease in currentdensity at a cathode of an electrochemical cell that includes asuppressed solution containing VMS and the one or more suppressor agentscompared to current density at a cathode of an electrochemical cell thatincludes a solution containing generally only the VMS, with each currentdensity measured at about −0.7 volts relative to a mercurous sulfateelectrode (“MSE”). For the electroplating solution 104 described herein,a suppressed solution includes the at least one acid and the at leastone suppressor agent. As merely an example, when a current density at acathode of an electrochemical cell utilizing a suppressed solution isfive times lower than a current density of an electrochemical cellutilizing a VMS, a suppressor agent provides a suppression strength of5.0.

As used herein, “acceleration strength” of one or more acceleratoragents of an electroplating solution 104 is measured by an increase incurrent density at a cathode of an electrochemical cell that includes anaccelerated solution containing VMS and the one or more acceleratoragents compared to current density at a cathode of an electrochemicalcell that includes the above-described suppressed solution, with eachcurrent density measured at about −0.7 volts relative to a MSE. For theelectroplating solution 104 described herein, an accelerated solutionincludes the at least one acid and the at least one accelerator agent.As merely an example, when a current density at a cathode of anelectrochemical cell utilizing an accelerated solution is two timeshigher than a current density of an electrochemical cell utilizing asuppressed solution, an accelerator agent provides an accelerationstrength of 2.0.

The at least one suppressor agent of the electroplating solution 104 isformulated to substantially suppress formation of dendrites duringelectroplating copper from the electroplating solution 104 and improveother qualities of an electroplated copper film, such as surfaceroughness, ductility, brightness, and electrical conductivity. The atleast one suppressor agent may be present in the electroplating aqueoussolution in concentration from about 10 mg/L to about 1000 mg/L. In someembodiments, the at least suppressor agent is present in theelectroplating solution 104 in an amount sufficient to provide asuppression strength of at least about 5.0. The at least one suppressoragent may be a surfactant, a leveler agent, a wetting agent, a chelatingagent, or an additive that exhibits a combination of any of theforegoing functionalities. The at least one suppressor agent may beselected from one or more of the following suppressor agents: aquaternized polyamine, a polyacrylamide, a cross-linked polyamide, aphenazine azo-dye (e.g., Janus Green B), an alkoxylated aminesurfactant, a polyether surfactant, a non-ionic surfactant, a cationicsurfactant; an anionic surfactant, a block copolymer surfactant,polyacrylic acid, a polyamine, aminocarboxylic acid, hydrocarboxylicacid, citric acid, entprol, edetic acid, tartaric acid, and any othersuitable suppressor agent.

When present, the at least one accelerator agent of the electroplatingsolution 104 is formulated to increase the deposition rate of copperonto the surface 112 of the substrate 110 and present in theelectroplating solution 104 in an amount sufficient to provide anacceleration strength of at least about 2.0. The at least oneaccelerator agent may further increase the brightness of theelectroplated copper film 120 and other qualities, such as decreasingvoid concentration in the electroplated copper film 120. The at leastone accelerator agent may be present in the electroplating solution 104in concentration from about 10 mg/L to about 1000 mg/L. According tovarious embodiments, the at least one accelerator agent may be selectedfrom an organic sulfide compound, such asbis(sodium-sulfopropyl)disulfide, 3-mercapto-1-propanesulfonic acidsodium salt, N,N-dimethyl-dithiocarbamyl propylsulfonic acid sodiumsalt, 3-S-isothiuronium propyl sulfonate, or mixtures of any of thepreceding chemicals. Additional suitable accelerator agents include, butare not limited to, thiourea, allylthiourea, acetylthiourea, pyridine,mixtures of any of the preceding chemicals, or another suitableaccelerator agent. The at least one accelerator may also comprise aninorganic compound selected to increase the deposition rate of thecopper from the electroplating solution 104, decrease hydrogen evolutionthat can increase the porosity in the electroplated copper film 120, orboth. For example, suitable inorganic compounds may compriseselenium-containing anions (e.g., SeO₃ ²⁻ and Se²⁻),tellurium-containing anions (e.g., TeO₃ ²⁻ and Te²⁻), or both.

Additionally, many of the disclosed accelerator agents may besubstantially-free of alkali elements (e.g., sodium and potassium),which can be detrimental to the performance of semiconductor devicesused in ICs. Accordingly, a copper film deposited from one of thedisclosed electroplating solutions having an accelerator agent that issubstantially free of alkali elements will also be substantially-free ofalkali elements.

For example, in an embodiment of the invention, a substantiallysodium-free accelerator agent of 3,3′-Dithio-1,1′-propanedisulfonic acidmay be synthesized. As shown in reaction (1) below, thiourea orN-substituted derivatives of thiourea having at least one hydrogen atomattached to one or both of the nitrogen atoms may be reacted with the1,3-propane sultone to form S-thiuronium alkane sulfonate, which is aderivative of thiourea containing a sulfonic acid group.

As shown below in reaction (2) below, S-thiuronium alkane sulfonate maybe reacted with an aqueous solution of ammonia to produce guanidinium3-mercapto-alkanesulfonate.

As shown in reaction (3) below, the quanidinium3-mercapto-alkanesulfonate so-formed may be passed through a cationicion exchange resin so that quanidinium ions are replaced by hydrogenions to form 3-mercapto-1-propanesulfonic acid.

Then, the 3-mercapto-1-propanesulfonic acid so-formed may be dissolvedin water in an amount, for example, to form a 10 percent by masssolution. Diethylamine (e.g., about 0.25 g/mol) may be added to thesolution. The mixture may be heated to reflux and while being mixed(e.g., by stirring) a small about (e.g., 0.05 g/mol) of powdered sulfurmay be added to the mixture. Then, the mixture may be refluxed for asufficient time (e.g., 8 to 10 hours) until the reaction is completed.The water may be evaporated in vacuum. The chemical reaction is shown inreaction (4) and the bis(diethylammonium)3,3′-dithio-1,1′-dipropanedisulfonate so-formed is typically a brownviscous syrup.

The bis(diethylammonium) 3,3′-dithio-1,1′-dipropanedisulfonate isdissolved in water to obtain, for example, a 10 percent by masssolution. As shown in reaction (5) below, this solution may be passedthrough an ion exchange resin of, for example, Amberlite IR-120 ionexchange resin operating in its acid cycle or another suitable ionexchange resin. After washing the ion exchange resin with water untilthe pH of the effluate is about 5 to about 6, the aqueous effluates maybe combined and evaporated in vacuum until substantially all the wateris removed from the reaction product, which is3,3′-Dithio-1,1′-propanedisulfonic acid in the form of a light-brownviscous syrup.

In another embodiment of the invention, an accelerator agent ofsubstantially sodium-free 3,3′-Dithio-1,1′-propanedisulfonic acid may besynthesized directly via an ion exchange process. For example,bis(sodium-sulfopropyl)disulfide may be passed through a suitable ionexchange medium to remove substantially all of the sodium and formsubstantially sodium-free 3,3′-Dithio-1,1′-propanedisulfonic acidaccording to reaction (6) below.

Referring again to FIG. 1, in other embodiments of the invention, theanode 106 may be configured as an inert anode, such as a platinum anode(e.g., a porous platinum anode) having a selected surface area relativeto the surface 112 of the substrate 110 being electroplated so that incombination with the chemistry of the electroplating solution 104 theratio of the cathode current density at the surface 112 to the anodecurrent density at the anode 106 is at least about 20. In such anembodiment, the electroplating solution 104 includes copper in the formof cupric ions (Cu²⁺) dissolved therein from another copper sourcebesides the anode 104. The copper may be present in the electroplatingsolution 104 in a concentration of at least about 50 g/L and, moreparticularly, from about 50 g/L to about 100 g/L. In a more specificembodiment of the invention, the concentration of the copper may be atleast about 75 g/L to about 100 g/L, and more particularly about 75 g/L.The copper may be provided from a copper source, such as one or more ofthe following copper sources: copper sulfate, copper polyphosphate,copper sulfamate, copper alkane sulfonate, copper chloride, copperacetate, copper formate, copper fluoride, copper nitrate, copper oxide,copper tetrafluoroborate, copper trifluoromethanesulfonate, coppertrifluoroacetate, copper hydroxide, and any other suitable coppersource.

One application of the above-described embodiments of electroplatingmethods is for electroplating copper to form bumps (also known aspillars) on a semiconductor substrate. FIGS. 5A-5E are cross-sectionalviews illustrating various stages in a method of electroplating bumpsaccording to an embodiment of the invention. Referring to FIG. 5A, inone embodiment of the invention, a semiconductor substrate 500 (e.g., asingle crystal silicon substrate) having a surface 502 is provided.Referring to FIG. 5B, a seed layer 504 may be deposited onto the surface502 of the substrate 500 using, for example, CVD, PVD, or anothersuitable deposition technique. For example, the seed layer 504 maycomprise tungsten, copper, or another suitable seed layer that promotesthe deposition of copper. Although not shown, an adhesion layer madefrom titanium, tungsten, alloys thereof, or another suitable materialmay be deposited onto the surface 502, and the seed layer 504 may bedeposited onto the adhesion layer to improve adhesion of the seed layer504.

Referring to FIG. 5C, a photoresist may be applied to the seed layer 504and photolithographically patterned to form a mask layer 506 having aplurality of openings 508 therein. Referring to FIG. 5D, copper may beelectroplated into the plurality of openings 508 and on the exposedportions of the seed layer 506 to form a plurality of bumps 510. Thecopper may be electroplated into the plurality of openings 508 and ontosurfaces 509 exposed through the mask layer 506 using any of thepreviously described embodiments of methods described herein. Thesurface area of the anode 106 and surface area of respective surfaces509 are selected in combination with the chemistry of the electroplatingsolution 104 so that the copper is plated under conditions in which aratio of a cathode current density at the respective surfaces 509 to ananode current density at the anode 106 is at least about 20.

The geometry of the bumps 510 may be selectively controlled by theconcentration of the accelerator agent and the at least one acid (e.g.,sulfuric acid) in the electroplating solution 104. In an embodiment, theelectroplating solution 104 may include an accelerator agentconcentration between about 10 to about 120 parts per million (“ppm”).The bumps 510 exhibit hemispherical-type geometry (i.e., convexlycurved) at high accelerator agent concentrations, a dimpled geometry atlow accelerator agent concentrations, and a substantially planar uppersurface (shown in FIGS. 5D and 5E) at accelerator agent concentrationsin between 10 ppm and 120 ppm. For example, for a given acceleratoragent concentration, a sulfuric acid concentration of 60 g/L in theelectroplating solution 104 may result in generally flat bumps 510,while a sulfuric acid concentration of about 30 g/L may result indimpled bumps 510.

Referring to FIG. 5E, the mask layer 506 may be stripped using asuitable solvent and exposed portions of the seed layer 504 may beremoved by etching.

Embodiments of Methods for Conditioning a Consumable Copper Anode

Referring to again FIG. 1, in another embodiment of the invention, theanode 106 may be a consumable copper-containing anode having a surfacearea of about two times or more than that of the surface 112 of thesubstrate 110 being electroplated. For example, the anode 106 may beconfigured as a generally flat plate comprised of copper and having asurface area of about two times or more than that of the surface 112 ofthe substrate 110 being electroplated. Unlike the above-describedembodiments, in this embodiment, the copper film 120 is notelectroplated onto the surface 112 under conditions in which a ratio ofa cathode current density at the surface to an anode current density atthe anode is at least about 20. In this embodiment, the anode 106 istreated to form a porous anodic film (not shown) that promotes theformation of a more uniform electrical double layer adjacent to itduring the electroplating of the copper film 120. That is, the porousanodic film of the anode 106 enables utilizing a high current density(e.g., about 400 mA/cm²) at the anode 106 while still maintaining arelatively uniform electrical double layer in the electroplatingsolution 104 adjacent it despite the anode 106 having a generally flatconfiguration.

The porous anodic film formed on the anode 106 may comprise copperoxide, copper chloride, copper phosphide, or combinations of theforegoing depending upon the chemistry of the electroplating solution104. Because the porous anodic film is porous, copper from the anode 106may still oxidize during the electroplating process and dissolve intothe electroplating solution 104, and plate onto the surface 112 of thesubstrate 110.

In practice, the porous anodic film may be formed on the copper anode106 by passing a charge of about 1000 coulombs per liter ofelectroplating solution 104 (“C/L”) or more through the electroplatingsolution 104 of the electrochemical cell 115 using a conditioningcathode instead of using the surface 112 of the substrate 110 as thecathode as depicted in FIG. 1. For example, the conditioning cathode maybe formed from a material that is relatively chemically inert in theelectroplating solution 104. A charge of about 1000 C/L or more may begenerated by applying a selected voltage waveform, using the powersource 114, between the surface 112 to be electroplated and the anode106 so that a current is passed through the electrochemical cell 115 fora time sufficient to pass 1000 C/L through the electroplating solution104 and form the porous anodic film.

After forming the porous anodic film on the anode 106, the conditioningcathode may be removed and replaced with the surface 112 of thesubstrate 110 to be electroplated. That is, the surface 112 of thesubstrate 110 functions as the cathode. Then, a copper film 120 may beelectroplated onto the surface 112, as previously described, by applyinga selected voltage between the surface 112 and the anode 106 having theporous anodic film formed thereon as a result of the conditioningprocess. For example, any of the previously described compositions forthe electroplating solution 104 may be used. Furthermore, the substrate110 may be moved (e.g., rotated, linearly oscillated, or both) duringelectroplating.

In an embodiment of the invention, the conditioning process for formingthe porous anodic film on the anode 106 may be performed in a separateconditioning electrochemical cell including the anode 106,electroplating solution 104, and the conditioning cathode. Then, theconditioned anode 106 having the porous anodic film formed thereon maybe removed from the conditioning electrochemical cell used in a separateelectroplating electrochemical cell, such as the electrochemical cell115 shown in FIG. 1, for electroplating the copper film 120.

Utilizing the disclosed electroplating methods in which the anode 106has been conditioned, the copper film 120 may be electroplated onto thesurface 112 of the substrate 110 at a high-deposition rate, such asabout 6 μm per minute or more and, more particularly, about 9 μm perminute. Additionally, the electroplated copper film 120 may besubstantially free of dendrites and exhibit a high-degree of (111)crystallographic texture that is more resistant to stress-inducedvoiding than other crystallographic textures.

The disclosed methods may be used for electroplating a high-qualitycopper film at a high-deposition rate to form many different types ofelectrically conductive structures other than copper bumps or pillars.For example, copper electroplated according to methods disclosed hereinmay be used to form interconnects for ICs using a Damascene process.Copper electroplated according to methods disclosed herein may also beused to form through-substrate interconnects or other metallizationstructures in ICs and other electronic devices. Moreover, copperelectroplated according to methods disclosed herein may also be used toform electrical contacts for solar cells.

The foregoing, non-limiting, list of applications merely provides someexamples of uses of copper electroplated according to the embodiments ofmethods disclosed herein.

WORKING EXAMPLES

The following working examples set forth methods for electroplatingcopper bumps and a method for synthesizing an accelerator agent composedof 3,3′-Dithio-1,1′-propanedisulfonic acid that is substantially free ofalkali elements. Examples 1 and 2 are working examples of methods forelectroplating copper bumps at a cathode current density to anodecurrent density ratio of greater than 20. Example 3 is a working exampleof a method for electroplating copper bumps using a flat copper anodethat has been conditioned. Example 4 is a working example of a methodfor forming an accelerator agent of 3,3′-Dithio-1,1′-propanedisulfonicacid that is substantially free of alkali elements. The followingworking examples provide further detail in connection with the specificembodiments described above.

Example 1

A 20 nm thick titanium adhesion layer was deposited onto a surface of asingle-crystal silicon wafer, followed by depositing a 100 nm thickcopper seed layer onto the adhesion layer. The titanium adhesion layerand copper seed layer were each deposited using PVD. A photoresist wasapplied to the seed layer and photolithographically patterned to form amask layer having a plurality of openings therein that exposed portionsof the seed layer. An electrochemical cell was formed by immersing thesilicon wafer including the mask layer in an electroplating solutionwith an anode. The anode of the electrochemical cell was made from agrooved copper anode. The electroplating solution had a composition of60 g/L of copper, 60 g/L of sulfuric acid, and 50 mg/L hydrochloricacid. The electroplating solution further included the followingadditives: 2 mL/L of eMAT™ accelerator/brightener RB10, 20 mL/L of eMAT™suppressor RS14, and 5 mL/L of eMAT™ leveler RL6.

Copper was electroplated into the plurality of openings of the masklayer and onto the exposed, respective surfaces of the seed layer byapplying a voltage between the anode and the silicon wafer to impress acathodic direct current density of about 300 mA/cm² to form a pluralityof bumps. The ratio of the current density at the cathode to the currentdensity at the anode was about 40. The copper was deposited at a rate ofabout 6 μm per minute. Examination of the electroplated copper bumpsusing a scanning electron microscope showed that the copper bumps weregenerally dendrite free, had relatively planar upper surfaces, andexhibited a surface roughness of less than 20 nm. X-ray diffraction alsoshowed that the copper bumps had a strong (111) crystallographictexture.

Example 2

A 20 nm thick Ti adhesion layer was deposited onto a surface of asingle-crystal silicon wafer, followed by depositing a 100 nm thickcopper seed layer onto the adhesion layer. The titanium adhesion layerand copper seed layer were each deposited using PVD. A photoresist wasapplied to the seed layer and photolithographically patterned to form amask layer having a plurality of openings therein that exposed portionsof the seed layer. An electrochemical cell was formed by immersing thesilicon wafer including the mask layer in an electroplating solutionwith an anode. The anode of the electrochemical cell was made from agrooved copper anode. The electroplating solution had a composition of60 g/L of copper, 60 g/L of sulfuric acid, and 50 mg/L of hydrochloricacid. The electroplating solution further included the followingadditives: 2 mL/L of eMAT™ accelerator/brightener RB10, 20 mL/L of eMAT™suppressor RS14, and 5 mL/L of eMAT™ leveler RL6.

Copper was electroplated into the plurality of openings of the masklayer and onto the exposed, respective surfaces of the seed layer byapplying a voltage between the anode and the silicon wafer to impress acathodic direct current density of about 700 mA/cm² to form a pluralityof bumps. The ratio of the current density at the cathode to the currentdensity at the anode was about 60. The copper was deposited at a rate ofabout 11 μm per minute. Examination of the electroplated copper bumpsusing a scanning electron microscope showed that the copper bumps weregenerally dendrite free, had relatively planar upper surfaces, andexhibited a surface roughness of less than 20 nm. X-ray diffraction alsoshowed that the copper bumps had a strong (111) crystallographictexture.

Example 3

A 20 nm thick titanium-tungsten alloy adhesion layer was deposited ontoa surface of a single-crystal silicon wafer, followed by depositing a100 nm thick copper seed layer onto the adhesion layer. Thetitanium-tungsten alloy adhesion layer and copper seed layer were eachdeposited using PVD. A photoresist was applied to the seed layer andphotolithographically patterned to form a mask layer having a pluralityof openings therein that exposed portions of the seed layer. Anelectrochemical cell was formed by immersing the silicon wafer includingthe mask layer in an electroplating solution with an anode. The anode ofthe electrochemical cell was made from a piece of flat copper. Theelectroplating solution had a composition of 60 g/L of copper, 60 g/L ofsulfuric acid, and 50 mg/L of hydrochloric acid. The electroplatingsolution further included the following additives: 2 mL/L of eMAT™accelerator/brightener RB10, 20 mL/L of eMAT™ suppressor RS14, and 5mL/L of eMAT™ leveler RL6. The copper anode was conditioned in theelectroplating solution by passing a charge of about 5000 C/L throughthe electroplating solution by impressing a direct current density of400 mA/cm² at the copper anode.

Copper was electroplated into the plurality of openings of the masklayer and onto the exposed, respective surfaces of the seed layer byapplying a voltage between the anode and the silicon wafer to impress acathodic direct current density of about 200 mA/cm² to form a pluralityof bumps. The ratio of the current density at the cathode to the currentdensity at the anode was about 4. The copper was deposited at a rate of7 μm per minute. Examination of the electroplated copper bumps using ascanning electron microscope showed that the copper bumps were generallydendrite free, had relatively planar upper surfaces, and exhibited asurface roughness of less than 20 nm. X-ray diffraction also showed thatthe copper bumps had a strong (111) crystallographic texture.

Example 4

Thiourea was reacted with the 1,3-propane sultone to form S-thiuroniumalkane sulfonate. The S-thiuronium alkane sulfonate so-formed wasreacted with an aqueous solution of ammonia to produce guanidinium3-mercapto-alkanesulfonate. The quanidinium 3-mercapto-alkanesulfonateso-formed was passed through a cationic ion exchange resin so thatquanidinium ions were replaced by hydrogen ions to form3-mercapto-1-propanesulfonic acid. Then, the3-mercapto-1-propanesulfonic acid so-formed was dissolved in water in anamount to form a 10 percent by mass solution. Diethylamine in an amountof about 0.25 g/mol was added to the solution. The mixture was heated toreflux and, while being mixed by stirring, about 0.05 g/mol of powderedsulfur was added to the mixture. Then, the mixture was refluxed forabout 8 to 10 hours until the reaction was completed andbis(diethylammonium) 3,3′-dithio-1,1′-dipropanedisulfonate was formed.The water was evaporated in vacuum. The bis(diethylammonium)3,3′-dithio-1,1′-dipropanedisulfonate so-formed was a brown viscoussyrup.

The bis(diethylammonium) 3,3′-dithio-1,1-dipropanedisulfonate wasdissolved in water to obtain a 10 percent by mass solution. Thissolution was passed through an ion exchange resin of Amberlite IR-120ion exchange resin operating in its acid cycle. After washing the ionexchange resin with water until the pH of the effluate was about 5 toabout 6, the aqueous effluates were combined and evaporated in vacuumuntil all the water was removed from the reaction product and3,3′-Dithio-1,1′-propanedisulfonic acid was obtained having alight-brown viscous syrup.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention.

1. A method, comprising: forming an electrochemical cell including atleast one surface of a substrate, an anode, and an electroplatingsolution in contact with the at least one surface and the anode, whereinthe electroplating solution includes at least one suppressor agent; andelectroplating copper onto the at least one surface under conditions inwhich a ratio of a cathode current density at the at least one surfaceto an anode current density at the anode is at least about
 20. 2. Themethod of claim 1 wherein electroplating copper onto the at least onesurface under conditions in which a ratio of a cathode current densityat the surface to an anode current density at the anode is at leastabout 20 comprises electroplating the copper onto the at least onesurface under conditions in which the ratio is about 20 to about
 100. 3.The method of claim 1 wherein electroplating copper onto the at leastone surface under conditions in which a ratio of a cathode currentdensity at the surface to an anode current density at the anode is atleast about 20 comprises electroplating the copper onto the at least onesurface under conditions in which the ratio is about 40 to about
 100. 4.The method of claim 1 wherein electroplating copper onto the at leastone surface under conditions in which a ratio of a cathode currentdensity at the surface to an anode current density at the anode is atleast about 20 comprises electroplating the copper onto the at least onesurface under conditions in which the ratio is about 60 to about
 100. 5.The method of claim 1 wherein electroplating copper onto the at leastone surface under conditions in which a ratio of a cathode currentdensity at the surface to an anode current density at the anode is atleast about 20 comprises: selecting a strength and a concentration ofthe at least one suppressor agent, and a surface area of the anode sothat the ratio is established during the act of electroplating copper.6. The method of claim 1 wherein: the anode comprises an inert anode;and electroplating copper onto the at least one surface compriseselectroplating the copper from dissolved copper in the electroplatingsolution.
 7. The method of claim 1 wherein: the anode comprises acopper-containing anode; and electroplating copper onto the at least onesurface comprises electroplating the copper provided from thecopper-containing anode.
 8. The method of claim 1 wherein the anodecomprises a copper-containing anode.
 9. The method of claim 8 whereinthe copper-containing anode comprises: a porous mass of copperparticles; a grooved body comprising copper; a porous mass of sinteredcopper-containing particles; or a mesh comprising copper.
 10. (canceled)11. The method of claim 1 wherein electroplating copper comprisesapplying a time-varying voltage between the anode and the at least onesurface of the substrate.
 12. The method of claim 1 wherein forming anelectrochemical cell including at least one surface of a substrate, ananode, and an electroplating solution in contact with the at least onesurface and the anode comprises forming the electrochemical cell toinclude the at least one surface, the anode, and the electroplatingsolution having dissolved copper therein present in a concentration fromabout 50 grams per liter to about 100 grams per liter.
 13. The method ofclaim 1 wherein the at least one suppressor agent of the electroplatingsolution comprises one or more of the following suppressor agents: asurfactant; a chelating agent; a leveler agent; and a wetting agent. 14.The method of claim 1 wherein the electroplating solution comprises atleast one accelerator agent.
 15. The method of claim 14 wherein the atleast one accelerator agent is substantially free of at least one typeof alkali element.
 16. The method of claim 1, further comprisinglinearly oscillating the substrate in the electroplating solution duringthe act of electroplating copper.
 17. The method of claim 1, furthercomprising rotating the substrate in the electroplating solution duringthe act of electroplating copper.
 18. (canceled)
 19. The method of claim1 wherein electroplating copper comprises depositing the copper onto theat least one surface of the substrate as a substantially dendrite-freefilm at a deposition rate of at least about 6 μm per minute.
 20. Themethod of claim 1 wherein the at least one suppressor agent provides asuppression strength of at least about 5.0.
 21. The method of claim 1wherein forming an electrochemical cell including at least one surfaceof a substrate, an anode, and an electroplating solution in contact withthe at least one surface and the anode comprises immersing the substratein the electroplating solution.
 22. A method, comprising: forming anelectrochemical cell including a cathode, a consumable copper-containinganode, and an electroplating solution in contact with the cathode andthe consumable copper-containing anode; and forming a porous anodic filmon the consumable copper-containing anode by generating a currentthrough the electrochemical cell for a time sufficient to pass at leastabout 1000 coulombs per liter of the electroplating solution through theelectroplating solution. 23-34. (canceled)